As Mark Segal points out, choosing the right back-up application depends on the particulars of your computing environment and the goals of your back-up strategy, and I would add that another factor to consider is your own software skills. If you are familiar with simple scripting techniques, or are willing to invest a little time in learning them, a commercial product or even a free one with a graphical user agent may not be the best choice—again, of course, depending on what you want to accomplish.
Many of the commercial products I've tried over the years (both in managing an enterprise IT infrastructure as well as running a fairly extensive home network) appear to be derivatives of open-source software, often based on standard UNIX/Linux utilities which run pretty much unaltered on Apple's OS X or have been ported to Microsoft Windows. The commercial products are sometimes easier to use—in many cases simply because the vendor has pasted a graphical front-end on top of what was natively a command-line program—but they're not necessarily more robust. In fact, they may be less so because (1) bugs in open-source software that is still being actively maintained are often fixed faster than those in commercial products and (2) the standard UNIX/Linux utilities tend to be remarkably reliable, even after porting to MS Windows.
Other than ease-of-use, it seems to me that an essential attribute to consider is how much data integrity-checking you require. The expensive enterprise back-up software products I used to license verified every file after it was written to the back-up device. Several different techniques can be used to accomplish this, but in general the idea was to guarantee that the copies were exactly the same as the originals so that transient hardware hiccups didn't leave us with a broken back-up. I don't recall ever seeing this in any consumer back-up application. One reason I'm partial to simple command-line utilities for home use is that if you have even a modicum of scripting experience it's fairly easy to build a wrapper around these programs that will verify the integrity of your back-ups. Similarly, you can use simple scripts to copy files simultaneously or sequentially to several back-up devices. My back-up philosophy is that lightning may indeed strike twice, so I consider three identical copies of any files I can't afford to lose to be a minimum.
